426 History of the English Landed Interest. 



means of restoring purity to air vitiated by the burning of 

 a candle in it, and this he was fortunate enough to find in 

 vegetation. The sprig of mint placed under an inverted glass 

 jar standing in water, though expected to fade, flourished. 

 This prompted him to attempt, by vegetable instrumentality, 

 the purification of air that had been injured by other natural 

 processes, such as animal respiration and putrefaction. Again 

 successful, he went on to examine the phenomena connected 

 with breathing.^ 



But before we can understand the conclusions arrived at 

 by Priestley in this direction, we must ascertain the bias of 

 mind in which he approached such experiments. He was to 

 his death a strong advocate of the phlogistic theory. This, 

 according to Becher, was an " inflammable earth," according 

 to Stahl, " a subtle principle," which belonged to any body 

 that was combustible. Phosphorus was a compound of phos- 

 phoric acid and phlogiston ; charcoal, because it left on com- 

 bustion very httle ash, was nearly pure phlogiston ; and metals, 

 because only slightly consumed when burned, were almost 

 entirely free from phlogiston. Now prior to his experiments 

 with air, Priestley had been heating oxide of mercury, and by 

 its sudden resolution into the pure metal had become possessed 

 of the secret of a new gas. He subsequently came across the 

 same element as one of the constituents of the atmosphere. 

 All these phenomena induced him to inquire, what part did 

 air play in the process of combustion ? The new gas Priestley 

 soon found to be a strong supporter of combustion ; was it on 

 that account rich in phlogiston ? Again, was the air a sup- 

 porter of combustion, and if so, was it rich in phlogiston? 

 Unfortunately, neither Becher nor Stahl had explained the part 

 played by air in combustion, nor had either accounted for, 

 though one at least knew of, the increase in the weight of 

 metals when buiaied in air. Priestley, however, soon found 

 that if he must reconcile all the properties, either of the new 

 gas or of air, with the phlogistic theory, he must materially 

 alter Stahl's rendering of it. He therefore suggested that 



^ Compare Philosoph. Trans., vol. Ixv. art. 38, and vol. Ixvi. art. 13, 

 1776. 



